Machine elements which are exposed to friction and high temperatures, for example piston rings, must have surfaces which are corrosion-resistant, wear- and seizure-resistant as well as scorch-resistant and must also have good sliding properties. For this the machine elements, in particular their contact surfaces, can be covered with wear-protection layers in the form of electrolytically deposited hard chromium layers.
To improve wear and seizure resistance, solids particles can be embedded in electroplated hard chromium layers. Electroplated hard chromium layers which have a network of cracks and in the cracks of which solids particles are embedded are described in DE 3531410 A1 and EP 0217126 A1. Particularly advantageous properties can be achieved by diamond particles with a size from 0.25-0.4 μm, embedded in the cracks of such an electrolytically deposited hard chromium layer, as described in WO 2001/004386 A1 and EP 1114209 B1.
Electroplated hard chromium layers can also be provided with a microstructure. Structured electroplated hard chromium layers which have particularly good tribological properties are known from DE 10255853 A1, WO 2004/050960 A1, DE 102004019370 A1 and WO 2005/108648 A2. A hard chromium layer the structure of which is cup-shaped and/or labyrinthine and/or columnar can be obtained according to this prior art through the composition of the electrolyte used during production and the low current yield of below 12% as specific method measures. This cup-shaped and/or labyrinthine and/or columnar surface structure provides outstanding sliding properties and very good emergency running properties, as the surface structure is characterized by good lubricant-retention capacity.
In order to combine the high wear-, seizure- and scorch resistances of the above-named chromium solids particles layer with the good tribological properties of a structured hard chromium layer, a chromium solids particles layer can be applied to the above-described hard chromium layer and a double layer thus produced. In this way the advantageous cup-shaped and/or labyrinthine and/or columnar structure of the structured hard chromium layer can be transferred to the chromium solids particles layer, even more wear-resistant in comparison with the structured hard chromium layer, i.e. continued by the chromium solids particles layer, and thus the very high wear resistance of the chromium solids particles layer can be combined with the tribological advantages of the structured hard chromium layer.
However, a disadvantage of this type of double coating is that the electrolytic deposition process is laborious and expensive because of the need to change the deposition conditions and electrolyte, an overall relatively thick coating must be applied to the machine element and the structure of the top chromium solids particles layer is frequently no longer as pronounced as in the previously applied structured hard chromium layer. Furthermore, double coating brings with it the risk that, with machine elements rubbing after prolonged running times, for example long engine running times of correspondingly coated piston rings, the upper layer is eroded and then the structured hard chromium layer lying below, which is particle-free, leads to increased wear and scorching.
Thus far it has not been possible to embed particles in a thus-structured chromium layer because crack-forming method measures for embedding particles could not be combined in a single process with structure-generating method measures.